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• A variable testing temperature was implemented in the dry-wet cycle test. • Effect of dry-wet ratio on mechanical properties and chloride corrosion resistance of concrete was investigated. • Higher dry-wet ratio exacerbates the deterioration of concrete performance. • Cumulative damage caused by variable temperature dry-wet cycle accelerates chloride erosion. Numerous factors influence the concrete performance in dry-wet cycle environment, with dry-wet ratio and temperature being particularly significant. In this paper, the effect of dry-wet ratios (1:3, 1:1, and 3:1) on the chloride corrosion resistance of concrete was studied, and the variable test temperature and constant test temperature were implemented in the dry-wet cycle. The deterioration law of chloride corrosion resistance of concrete was investigated, and the microstructure, phase composition, and pore structure were examined. The results showed that the deterioration degree of concrete performance was intensified by a higher dry-wet ratio. Compared with D1W3 (the dry-wet ratio is 1:3), the compressive strength, splitting tensile strength, flexural strength, mass, and relative dynamic elasticity modulus (RDEM) of D3W1 (the dry-wet ratio is 3:1) exposed to 72 dry-wet cycles decreased by 11.05%, 19.40%, 8.33%, 1.09%, and 6.14%, respectively, while the peak chloride content and erosion depth increased by 40.98% and 22.75%. Compared to the dry-wet cycle with constant temperature, the cumulative damage caused by the dry-wet cycle with variable temperature was greater, resulting in more severe deterioration of concrete performance. Variable temperature accelerated the diffusion rate and penetration depth of chloride ions. The peak chloride content and erosion depth of D1W3 were 27.08% and 18.44% higher than those of control group after 72 dry-wet cycles. Microscopic analysis confirmed that higher dry-wet ratio and variable temperature cycles accelerated the decay of mechanical strength and the decline of chloride erosion resistance. [ABSTRACT FROM AUTHOR]

• The use of graphene and carbon fiber as conductive phase materials for the preparation of graphene-conductive asphalt-recycled permeable concrete. • Determined the optimal conductive filler ratio and achieved a resistivity of only 3.2 Ω·m. • The effect of the required snow melting thickness and snow melting time on the efficiency of pervious asphalt slabs for snow melting are studied. Conductive-asphalt-recycled pervious concrete alleviates the urban "heat island effect" and "rain island effect" to realize the resourceful and efficient use of waste concrete and also reduces the probability of urban snow and ice disasters to ensure the safety of winter operation of asphalt pavements in cold areas. This study presents a theoretical and technical basis for ensuring the winter operational safety of asphalt pavements in cold regions. First, we compound graphene and carbon fiber as conductive phase materials and prepare graphene-conductive asphalt-recycled permeable concrete using recycled coarse aggregates with straight top and bottom holes. Then, we analyzed the electrical conductivity, road performance, and electrothermal performance of the specimens prepared using different recycled coarse aggregate substitution rates under different snowfall levels to determine their snow melting efficiency and verify their application in cold regions. The results indicate that the quality of the recycled aggregates is high, the optimal conductive filler ratio is 0.3% polyacrylonitrile-based carbon fiber and 1.5% graphene by mass of asphalt, and the resistivity of the specimens is only 3.2 Ω·m. Moreover, the conductivity and road performance of the graphene-conductive asphalt-recycled concrete decreases with an increase in the replacement rate of the recycled coarse aggregate, and the artificially prefabricated upper and lower through holes affect the conductivity of the specimens to a certain extent. Further, the graphene-conductive asphalt-recycled pervious concrete shows excellent snow melting potential, and as the required snow melting thickness and snow melting time increase, the effect of pervious asphalt slabs for snow melting efficiency becomes more significant. Moreover, when the replacement rate of the recycled concrete aggregate is within the range of 0–50%, it can be completed in 2–3 h under different snowfall levels of snow melting requirements. [ABSTRACT FROM AUTHOR]

• It is feasible to recycle waste RAC in harsh environments by carbonation treatment. • Accelerated carbonation shows excellent repair effect on the damage of RRCA. • Producing C-RRCA per ton can capture 38.3 kg CO 2 with great environmental benefits. • RRAC with C-RRCA (content ≥ 6 %) shows good chloride ion permeability resistance. The recycling of waste recycled concrete (RC) has become a focus of many scholars. But the repeatedly recycling of RC used in harsh environments is still a difficult issue, due to the poorer quality of repeatedly recycled aggregate. This paper studied the repair effect of accelerated carbonation treatment on repeatedly recycled coarse aggregate (RRCA), which was collected from the failed parent recycled concrete via coupling action of high stress and freeze-thaw (F-T) cycles. Then the properties of carbonated repeatedly recycled aggregate concrete (C-RRAC) containing 0 %, 25 %, 50 %, 75 % and 100 % carbonated RRCA (C-RRCA) were evaluated. And the recycling potential of C-RRCA was explored, finally. The results suggested that accelerated carbonation treatment had excellent ability on repairing coupling damage of high stress and F-T in RRCA. Compared with RRCA, the apparent density of C-RRCA increased by 5.5 %, and water absorption, crushing value and soundness (mass loss) decreased by 26.6 %, 23.7 % and 19.9 %, respectively. The quality of C-RRCA was improved from type III to type II. With the growth of C-RRCA replacement rate, the water absorption, porosity, and chloride ion permeability coefficient of C-RRAC decreased by 34.4 %, 48 % and 55.6 %, respectively, and the compressive strength increased by 34.2 %. In addition, the C-RRAC could satisfied requirement of 100 years design service life of concrete structures in the E-type chloride environment, totally. According to the numerical fitting analysis, the minimum content of C-RRCA is 6 % under the condition that the compressive strength and chloride ion permeability of recycled aggregate concrete are met. It is feasible to recycle waste or even failed RC in high stress and F-T environment so as to reuse it in chloride environment, which provides a data support for effective utilization of waste RC under freeze-thaw environment. [ABSTRACT FROM AUTHOR]

• The feasibility of recycling waste RAC in freeze–thaw environment is evaluated. • Freeze-thaw damage of RAC adversely affect the properties of second-generation CRA. • Freeze-thaw damage threshold of waste RAC as CRA for structural concrete is 0.36. • RAC with freeze–thaw failure cannot reuse as CRA for structure concrete. • Adhesive mortar and ITZ are the main carriers of freeze–thaw damage transfer. The reuse of waste recycled aggregate concrete (RAC) from freeze-thaw (FT) environment as coarse recycle aggregate (CRA) for structural concrete can effectively improve the application of second-generation CRA. This paper evaluated the reuse potential of waste RAC suffered different FT cycles as CRA for structural concrete by FT damage. C40 RAC specimens with 100% replacement rate were produced and the FT damage through a coupled action of FT cycles and 35% bending load was assessed. The RAC were crushed into second-generation CRA after every 100 FT cycles until FT failure. Results showed that the evolution process of FT damage on physical properties of CRA can be divided into four stages: reinforcement stage, transition stage, weakening stage and failure stage. In the evolution process, the apparent density of second-generation CRA decreased with FT cycles, while the crushing value, water absorption and soundness (mass loss) increased. The particle size of second-generation CRA gradually decreased with the accumulation of FT damage, and the screening curve of the CRA after 400 FT cycles cannot meet the requirements of continuous particle size. Relative dynamic modulus of elasticity (RDME) was more sensitive than mass loss rate in assessing the frost resistance durability of RAC. The FT damage threshold of RAC based on the properties requirements of type II and type III CRA was 0.085 and 0.359, respectively, indicating that the FT damage threshold of waste RAC crushing as CRA for structural concrete was 0.36, and the corresponding RDME was 64% rather than 60%. The microscopic analysis showed that the weakening of adhesive mortar and ITZ was the fundamental reason for the decrease of physical properties of the second-generation CRA. The CRA obtained from the RAC with FT failure cannot be reuse as aggregates for structure concrete. [ABSTRACT FROM AUTHOR]

• Utilization of RGFA as an alternative material for river sand is feasible. • RGA mortar showed comparable properties compared with NA mortar. • Unreacted metakaolin in RGFA was activated by alkali solution anew. • High bond strength of GP-RGFA ITZ lowered the decline tendency of CS. Geopolymer has been viewed as a potential, ideal and green material to replace cement. The recycling of Portland cement-based concrete as recycled aggregates was investigated diffusely, while the reuse of geopolymer was paid little attention. Exploring the feasibility and interfacial evolution of utilizing recycled geopolymer fine aggregate (RGFA) produced from waste geopolymer as the alternative for river sand (RS) was the core object in this research. Metakaolin to RS ratio of 1:2 and NaOH/Na 2 SiO 3 ratio of 0.5 by weight, and 12 M of NaOH solutions were adopted to prepare the referenced geopolymer mortar. Then five RGFA replacement rates (0%, 20%, 50%, 80% and 100%) were used to prepare the NA (RGFA) mortar. The tests of compressive strength, flexural strength and water absorption of mortars were performed. SEM was used to observe the interfacial transition zone (ITZ) between RGFA and new generated geopolymer paste. Results proved that RGFA mortar exhibited comparable mechanical properties compared with the RS mortar when replacement rate was less than 50%. Furthermore, more compact ITZs were obtained in RGFA mortar. The decreasing mechanical properties of RGFA mortar was primarily attributed to the weak properties of RGFA itself. The filler effects and compact ITZs relieved the decline tendency of mechanical property of RGFA mortar. It was convinced that utilizing RGFA to replace RS as fine aggregates in the construction activities was promising. [ABSTRACT FROM AUTHOR]

• An innovative ACP was prepared applied as tunnel fire resistant coating. • ACP has a lower thermal conductivity, excellent mechanical and durable properties. • ACP can defend effectively SCC from spalling in tunnel fire accidents. • 40 mm thickness of ACP can effectively withstand tunnel fire up to 2.5 h. Self-compacting concrete (SCC) widely used in tunnel structure is prone to spalling when it is subjected to the fire disaster. The fire-resistant materials coated on the surface of concrete are considered as an effective way to prevent concrete from being destroyed in a fire accident. However, current these coatings are difficult to withstand a high temperature, for example, tunnel fire temperature, and a longer-term fire. In this paper, a 40 mm thickness of SiO 2 aerogel cement paste (ACP) coating was prepared to investigate the protective effect for SCC in a simulated tunnel fire with the maximum temperature of 1100 °C for 2.5 h. The mechanical properties, thermal conductivity and fire resistance of SiO 2 ACP, and the compressive strength of SCC with or without ACP coating in the simulated tunnel fire were tested. The results showed that ACP exhibited excellent mechanical and durable performances and low thermal conductivity. The 40 mm thickness of ACP has a strong thermal insulation capacity and can effectively withstand tunnel fire up to 2.5 h. The SCC samples under the protection of ACP did not undergo compressive strength degradation and destruction of microstructure. The interfacial temperature of ACP and SCC was much lower than the critical value (380 °C) in a 2.5 h tunnel fire. Therefore, ACP can provide a good choice for protection of SCC in a tunnel fire with a high temperature and long time. [ABSTRACT FROM AUTHOR]